No. I have tried the adapter on a MacBook. This adapter with a MacBook only allows 4K 30Hz HDMI 2.0.

The MacBook is not one of the computers listed as supporting 4K 60Hz in the the Apple Support document HT207806 "About the Apple USB-C Digital AV Multiport Adapter".

The MacBook cannot output two lanes of DisplayPort 1.4 because the Intel graphics only supports DisplayPort 1.2.

Search for an adapter that does not include 5 Gbps USB 3.0 which will then allow four lanes of DisplayPort. For example, adapters that use the MCDP2900 chip will have four lanes operating at 5.4 Gbps per lane which gives them 33% more bandwidth than this adapter that uses a chip that has two lanes at 8.1 Gbps.

This adapter has 50% more bandwidth than the previous adapter but only some computers can use the new bandwidth. Read the Apple Support document HT207806 "About the Apple USB-C Digital AV Multiport Adapter" to see if your computer is one of them. Basically, you need to be able to output a two lane DisplayPort 1.4 signal (though I don't know how some of the listed computers can do that).

Another option is to reduce blanking times so that pixel clock/bandwidth can be decreased for the same refresh rate. Displays require a certain amount of blanking time though. For example, some displays may accept a 4K 60Hz 533 MHz timing.

If this adapter is not sufficient then search for one that does not include 5 Gbps USB 3.0 which will then allow four lanes of DisplayPort. For example, adapters that use the MCDP2900 chip will have four lanes operating at 5.4 Gbps per lane which gives them 33% more bandwidth than this adapter that uses a chip that has two lanes at 8.1 Gbps.

No for two reasons: firstly, this adapter only provides video output from USB-C to HDMI, not input from HDMI to USB-C. Secondly, regardless of the adapter, the 4K/5K iMacs do not support Target Display Mode, so you cannot do this on the 4K/5K iMac with *any* adapter, the only thing you can show on the iMac screen is the iMac itself.